Integrated react-router

For #1 . Feel free to ask any questions. Decided against `react-router-bootstrap` in the end because it was never really finished, won't ever be and we can do without. `react-router` has a flexible and well-documented API:  https://github.com/rackt/react-router/tree/master/docs/api
What is the dangerous part? I made some research and found that this was the way to explicitely send the static `index.html` file over and that it leveraged `express.static`. How do you otherwise trigger the sending of `index.html` in a middleware? 

I agree however that the `res.locals` tracking is cumbersome and totally unnecessary. The `isFrontEnd` middleware in routes could return the `index.html` itself and this would only handle 404s.

In the end, technically, all the static files this service will serve are `index.html` and `favicon.ico`, all others will be hosted on S3.